The present invention concerns a photographic support material with a coating on each side, at least one of the coatings comprising a mixture of polyolefins and optional or auxiliary agents, white pigments for example, that have a positive effect on the utilization of the support material.
The procedure of coating papers with thermoplastics for use as support materials for photographic coatings is known. Of primary interest to the manufacturer are water resistance, dimensional stability, and economy. Some of the coatings are polyolefins and olefin copolymers conventionally coated on both sides of the paper by melt extrusion. The "base paper" of the support material is primarily natural paper but can be a mixture of natural paper and plastic or a synthetic paper instead. Hereinafter, all of these types of paper are called "base paper".
The base paper employed in accordance with the invention in no way differs from those conventionally employed; for example, base paper with a basis weight of 170 g/m.sup.2. It will preferably have a coating of hydrophobic resin (usually a polyolefin) on each side. The hydrophobic coating nearest the coatings that incorporate the photographic image will contain at least one light-reflecting pigment. The support material is therefore protected from the entry of water or photographic developing solutions, but will, of course, be unprotected from the entry of aqueous developing solutions at its edges. To counteract this, the support material is usually "hard-sized". Both such reactive substances as alkyl-ketene dimers and such non-reactive substances as the more complex fatty acids are well known sizes for photographic support material.
The reactive sizes are usually treated in the neutral range (neutral sizing) with a cationic resin. The non-reactive sizes are employed in the acidic range with aluminum ions added to them.
A combination of both sizing procedures is also known, for example, from the German Patent Publication No. OS 3,238,865.
The sizes are hydrophobic. They are introduced into the pulp and are precipitated onto the fibers by added agents. One of the agents employed with an acidic size for example is an aluminum salt. A cationic polyamide-polyamine epichlorohydrin resin can be employed with a neutral size. Such other substances as cationic polyacrylamides, starches, and polyethylene imines, however, are also appropriate in specific cases to help retain reactive sizes within the support material.
Since the hard sizing of the support material occasions a loss of strength, substances that will make it stronger are also added to the pulp. Examples of such substances are polyvinyl alcohol, anionic polyacrylamide, and various starch derivatives.
In addition to interior hydrophobing, the surface of the support material can be sized with complex-molecular substances. Complex-molecular substances that can be employed for this purpose include gelatins, oxidized starches and other starch derivatives, carboxymethylcellulose, polyvinyl alcohol, etc.
The surface of the support material is coated with an aqueous mass containing such agents as brighteners, pigments, cross-linking agents, etc., in addition to the aforesaid complex-molecular substances.
Both pulp sizing and surface sizing are detrimental to the adhesion of the polyolefin coating applied later to the sized support material. A coating can accordingly be extruded over the surface only at a rate that usually does not exceed 150 to 180 m/min.
A waterproof support material for light-sensitive photographic coatings with a polyolefin coating on one or both sides is known, for example, from the U.S. Pat. No. 3,411,908. Also known, for example, from the European Patent Publication No. 290,852 are support materials with coatings of a polyolefin mixed with a copolymer of ethylene and another .alpha.-olefin. Another known support material has a coating comprising just pigment in an ionomeric resin consisting of .alpha.-olefines and .alpha.,.beta.-unsaturated monocarboxylic acids (see German Patent Publication No. OS 1,572,189).
A light-sensitive layer containing a silver halide is then applied to one of the plastic layers. The light-sensitive layer can be either a black-and-white or a color photography layer. Particular demands in relation to reflectivity, whiteness, smoothness, and sensitometry are made of the coating that the light-sensitive layer is applied to. The coating will accordingly contain such agents as white pigments, lubricants, antioxidants, brighteners, emulsifiers, antistatics, etc.
The subsequent application of layers of such hydrophilic substances as adhesion promoters containing gelatins and light-sensitive materials, and the use of aqueous photographic solutions for developing and fixing, demand that the coating adhere firmly tight to the support material. The trimmed edges of the photographic support material are particularly at risk because processing chemicals can penetrate into the unprotected material. When the adhesion between the coating and the support material is weak, they will delaminate and the edges will become ragged. Developer can also invade the support material at these areas and discolor it.
Various measures for improving the adhesion of a mainly polyolefin coating to a support are known. Increasing the temperature of the molten extruded polyolefin for example can increase the adhesion. Too high extrusion temperatures, however, are detrimental, especially in the presence of titanium dioxide as a white pigment. This substance is catalytically active and can accelerate decomposition of the resin. The decomposition can appear in the form of either yellowing or cloudiness due to the presence of breakdown products.
Another approach to improving adhesion is to subject the web to a corona. The U.S. Pat. Nos. 3,411,908 and 3,501,298 and the British Patent No. 1,005,631 for example, propose treating the support material with a corona to improve the adhesion of the polyolefin to it. Coronas, however, have several drawbacks. One is the relatively expensive high-voltage equipment. Another is that too powerful a corona will damage the support's internal sizing, exposing the trimmed edges to the developer.
Adhesion can also be improved with chemical adhesion promotors. Among these substances are copolymers of ethylene and vinyl acetate or acrylamide, ionomers, terpolymers, etc. The adhesion promotor is applied as an aqueous dispersion that forms an intermediate layer on the base paper. Such adhesion promotors are described for example in the German Patent No. 2,326,759 and the Japanese Patent Publication No. 49-15423. They usually do not ensure optimal adhesion. They entail an additional step that involves additional equipment. And the photographic quality of the overall support material can again be negatively affected.
Adhesion can also be improved by mixing synthetic fibers into the pulp. This approach is described in the German Patent No. 2,344,367 for example. The drawback is that a special pulp must be mixed.
It is also known that photographic base papers can be extrusion coated at only limited rates, of approximately 150 to 180 m/min.